High-pressure discharge lamps having a fill which includes a noble gas, for example xenon, have relatively high operating currents. Even lamps of medium power, for example several hundred watts, require relatively heavy electrodes, made of high melting material, typically tungsten. These electrodes are heavy. They are usually retained on rods or shafts which form current supplies to the electrodes, which have a diameter of several millimeters. These electrode shafts, also of tungsten, are melt-sealed through neck portions joined to the actual discharge vessel. Usually, the assembly is manual, hence expensive and complex. To match the thermal coefficient of expansion of tungsten to the much smaller coefficient of quartz glass, transition glasses of intermediate thermal coefficients of expansion are used between the tungsten and the quartz glass, to permit a gas-tight/"graded seal" melt connection of the passage of the electrode shafts through the wall of the neck portion of the discharge vessel.
The melts forming the through-connections are mechanically sensitive. No forces must be transferred thereto from the electrode rods, since cracks or fissures may result, which, then, leads to the leakage of the lamp, rendering it useless.
Throughout the development of these lamps, many proposals were made to improve the gas-tight seal of the tungsten electrode shaft or electrode rod through the quartz glass vessel. German Utility Model Publ. GM 1 939 204, filed in 1964, describes a noble-gas high-pressure discharge lamp in which, starting at the transition from the actual discharge bulb to the neck until close to the melt seal are constricted to form a portion of reduced diameter of capillary form. The constriction is obtained by deformation, for example by using detonating gas, such as an oxyhydrogen blow system, for heating the constriction to a temperature of about 2OOO.degree. C., thus softening the neck portion and permitting deformation. Providing a vacuum within the bulb structure facilitates engagement of the quartz glass on the electrode rods. Upon cooling to room temperature, a ring gap will result, having a spacing of a few tenths of a millimeter between the electrode rod and the supporting glass capillary. The capillary ensures reliable support of the electrode rods under the operating conditions of the lamp and provide for transfer of forces which may occur upon handling or transport, and which may arise due to vibration of the heavy electrodes. Forces on the melt seal itself are effectively prevented.
German Utility Model Publication GM 78 35 279, filed in 1978, illustrates a somewhat similar lamp having a support capillary structure of reduced length, and positioned in the bulb neck close to the bulbous portion. Reducing the length of the capillary improved the mechanical stability of the lamp, since shortening the capillary reduced the danger of breakage thereof. It further facilitated evacuation of the lamp, since the pumping resistance was reduced due to the shorter ring gap. This lamp was also substantially cheaper to make since a portion of the expensive manually made deformation work on the lead-through seal, which required highly skilled workers, was no longer needed.
U.S. Pat. No. 4,463,281, having a first filing (priority) date of Aug. 6, 1980, describes a high-pressure discharge lamp and a method of its manufacture, in which each one of the neck portions is formed with only a slight constriction in the vicinity of the transition between the neck portion and the bulbous portion of the discharge vessel. A support element is loosely threaded on the electrode shaft and engages the constriction. This support element is in the shape of a circular cylindrical roller of quartz glass, having a length of only a few millimeters. The support element is resiliently pressed against the constriction by a tungsten spring, threaded to surround the electrode rod and engaged against the melt seal. The electrode rod is thus supported resiliently and the reduced capillary, previously described, can be used to best advantage.
The slight constriction of the vessel neck reduces practically all danger of breakage in that region. The pumping path along the support element is reduced to only a few millimeters, and the previously expensive and complex deformation work is reduced to a minimum, merely to forming a slight constriction in the transition region between the discharge bulb and the neck portion of the overall discharge vessel.
The space which must be filled with noble gas has a relatively high volume, since it includes the neck portion beyond the bulb itself. This noble gas remains relatively cool in the neck portions, since it is not directly heated by the discharge process. Convection results, which lowers the overall temperature, and thus the operating pressure of the noble gas within the discharge bulb itself.
To obtain operating characteristics of lamps with capillaries which have not been foreshortened, it was necessary in the constructions so far described to increase the cold fill pressure of the noble gas, which then raises the necessary firing or ignition voltage required for firing or igniting the lamp to an undesired level. Additionally, costs arise because of the larger quantity of noble gas than absolutely necessary for operating of the lamp. The noble gas, typically xenon, is expensive.
It has been found that under some extreme and worst-case conditions, forces can be transmitted between the support element and the interior of the constriction, particularly during transport or careless handling. Under such conditions, friction, rubbing, and damage to the surface of the quartz glass may occur, undesirably affecting operation of the lamp. Resonance effects also may occur, particularly during transport.
In operation of the lamp, possible gas convection flow may lead to decrease of the stability of the discharge arc.
U.S. Pat. No. 4,559,472, having a first filing (priority) date of 1982, is a further step in the development of this type of lamp, and describes an electrode support system which eliminates the disadvantage of a relatively large volume which must be filled with noble gas within the portion of the discharge vessel neck, remote from the actual discharge space or bulbous discharge portion. An elongated circular cylindrical element, for example of quartz glass, is used which is loosely fitted on the electrode rod or shaft and fills the entire bulb neck between the melt seal and the actual discharge space or bulb itself. A spiral spring of tungsten loosely fitted on the electrode rod between the electrode and the quartz glass element appropriately positions this quartz glass element.
It has been found in actual use that it was difficult to properly match manufacturing tolerances and spring force to reliably position the relatively heavy quartz glass support element. Weakened springs and tolerance related variations resulted, from time to time, in damage to the melt seal by the heavy quartz element, movable along the electrode rod in axial direction.